The present invention pertains to an improvement in the formulations of anaerobic glues and adhesives.
It is known that various materials, especially metallic materials, can be joined using adhesives based on dimethacrylates that are formulated in such a manner that the hardening of these adhesives occurs as soon as they are removed from contact with the air. It is then sufficient to coat the surfaces to be joined with such an adhesive and to bring them together tightly, which induces polymerization of the adhesive and formation of a definitive bond.
These adhesives, which are often referred to as "anaerobic resins", are very widely employed in the field of mechanics to achieve the retention and tightness of threaded pieces such as screws, nuts, bolts, pins, etc. They also have an important role in the joining by attachment of small pieces (for example, for the attachment of wheels, pulleys or ball bearings on their axles) in which they advantageously replace the traditional techniques such as press assembly, banding and brazing and eliminate the need for supplementary joining pieces (keys, pins, washers and locking screws, etc.).
These anerobic adhesive compositions are based, in a very conventional manner, on a polyacrylic ester or a mixture of polyacrylic esters capable of radical polymerization at room temperature, such as tetraethylene glycol dimethacrylate, combined with substances capable of initiating the radical polymerization of the said polyacrylic esters by generation of free radicals as soon as the mixture is removed from the action of oxygen and of maintaining or accelerating this polymerization, traditionally an organic peroxide or hydroperoxide such as cumyl hydroperoxide combined with dimethylparatoluidine. Such base formulations are described in U.S. Pat. No. 3,218,305. They were subsequently improved by the addition of oxidation inhibitors, metallic complexing agents and viscosity additives, etc.
The base formulations set relatively slowly and attempts have been made to resolve this drawback by means of a suitable selection of the accelerator. For this purpose, use has been made of highly diverse nitrogen compounds, notably hydrazine derivatives such as, for example, ethyl carbazate or N-aminorhodanine (FR 1,580,572), acetylphenylhydrazine (FR 2,311,832), para-nitrophenylhydrazine (FR 2,283,191) or para-tolysulfonylhydrazide (FR 2,283,190). The most commonly obtained results are an appreciable increase in the setting speed on ferrous materials but at the price of an often noteworthy loss in the storage stability of the product and a real difficulty in obtaining polymerization of the adhesive on nonferrous surfaces or treated surfaces such as galvanized or bichromated steel.